The finding supports a theory that Antarctic sealing and whaling led to a krill population explosion, and the penguins apparently took advantage of the surplus.

"We have some pretty good direct data that penguins were switching to krill when it became highly available," said Steve Emslie, a biologist at the University of North Carolina in Wilmington.

Emslie is the co-author of a paper on the findings reported earlier this month in the journal Proceedings of the National Academy of Sciences.

Penguin Fossils

Emslie and colleague William Patterson of the University of Saskatchewan, Canada, based the finding on analysis of more than 220 fossil eggshell fragments ranging from the past 100 to 38,000 years.

The proportions of certain forms of carbon and nitrogen in the eggshells tell the researchers what type of food the penguins ate in the days before laying their eggs.

For most of the past 38,000 years, the penguins ate a variable diet heavy on fish, which are higher up the food chain than krill. The birds changed to krill a few centuries ago.

"This implies a huge ecological dietary response by the penguins in relation to some change in their environment," Keith Hobson of Environment Canada, the nation's environment agency, commented in an email.

The dietary shift coincides with the advent of whaling and sealing in the region, suggesting the sudden abundance of easier-to-catch krill was a preferable high-energy food source, Emslie said.

Hobson, who was not part of the research, said such an explanation is plausible but only makes sense if accompanied by a reduction in fish supply.